Commercial Photography sells a product.
Fashion Photography sells a lifestyle.
Editorial Photography sells a story.
Contrary to commercial photography, the emphasis in fashion photography does not lie on the garments, but on the mood and styling of the image. The garments are merely an accessory to convey a certain lifestyle. Therefore, the whole image is much more complex. You usually won’t see plain white or off-white backgrounds in fashion photography and the models are usually styled very dramatically with thick eyeliners, dramatic eye shadows, etc. With the increased complexity in styling also comes an increased complexity in lighting to make the image look more dramatic. While many commercial photographers only stick with one light for the model, fashion photographers tend to use a wide array of lights and lighting accessories. When doing a fashion shoot, I have used up to seven lights to get the dramatic look that I wanted, but I know photographers who use ten or more lights for fashion. That is not to say that a dramatic look can not be achieved with only one light, such as a beauty dish or an octabank – it’s about the effect of the lighting and not about how many lights you use to get that effect.